Follicle-Stimulating Hormone (FSH) stimulates gametogenesis. Besides this hormone vitamin E is also essential for gametogenesis. Deficiency of vitamin E leads to sterility. Vitamin A is also required for the formation of healthy gametes.
Gametogenesis is divided into three stages:
As there are two types of gametes (Spermatozoa and Ova), gametogenesis can be studied under two broad headings:
Spermatogenesis is the formation of spermatozoa, whereas oogenesis is the formation of ova.
Both spermatozoa and ova originate from primordial germ cells(PGCs), which are extra-gonadal in origin.
In humans, the PGCs originate during early embryonic development from the extra-embryonic mesoderm. Eventually, they migrate to the yolk sac endoderm, and ultimately, to the gonads of the developing embryo, where they undergo further development.
You can recall that spermatogenesis occurs in the seminiferous tubules of the testes and oogenesis occurs in the follicles of the ovary. The formation of gametes starts at puberty.
Question 1:Polar bodies are formed during
Polar bodies are smaller cells produced during oogenesis that do not develop into egg cells.
Spermatogenesis, i.e. formation of sperms. In most animals, spermatogenesis takes place in testes (exception-earthworm). Mammalian testes contain seminiferous tubules and the wall of the seminiferous tubule is composed of germinal epithelium.
It contains some special types of cells called primordial germ cells and these cells start spermatogenesis. On the basis of origin, primordial germ cells are extra-embryonic mesodermal.
Besides these cells, germinal epithelium contains some large-sized cells called Sertoli cells. The occurrence of Sertoli cells is the unique feature of mammalian testis.
Sertoli cells provide nutrition to developing sperm, i.e. developing sperms are embedded in the cytoplasm of Sertoli cells and absorb nutrition after maturation sperms come out from Sertoli cells and liberate in seminiferous tubules.
The liberation of sperms from Sertoli cells is called Spermination.
The liberation of sperms from testes is called Semination.
The liberation of sperms from the body of the male is called Ejaculation.
Mammalian sperms are transferred to the vagina of females by the process called insemination.
Sertoli cells form the 'Blood testes barrier' and protect the sperm from the body's immune system because antibodies may attack haploid cells and destroy them. (Sperms are haploid and other cells of the body are diploid).
Question 2:Which of the following cells during gametogenesis is normally diploid ?
Spermatogonia are diploid cells which mature into primary spermatocytes (2n) by growth. They then produce two haploid secondary spermatocytes by meiosis I. Each secondary spermatocyte (n) completes the meiosis II and produces two spermatids (n). Each spermatid (n) develops into a spermatozoan or sperm (n). Similarly, in females, oogonia are the diploid cells from which through meiosis, polar bodies (n) and single ovum (n) are produced.
Sertoli cells function as an endocrine gland. i.e. secrete three types of hormones:
(i) Antimullerian hormone: This hormone's function is the degradation of female gonads in the male embryo (In male seminal vesicle is the ruminant part of the oviduct of female).
(ii) Inhibin hormone: This hormone's function is to control excess secretion of the pituitary gland and prevent the over-production of sperms.
(iii) Androgen binding protein: This hormone's function is to concentrate testosterone in seminiferous tubules because testosterone is a must for spermatogenesis in seminiferous tubules.
Spermatozoa are formed in the wall of the seminiferous tubules of the testes. The various cell stages in spermatogenesis are as follows (the number of chromosomes at each stage is given in brackets):
The spermatogonia (type A) or germ cells (44 X+Y) divide mitotically, to give rise to more spermatogonia of type A (spermatogenic lineage) and also spermatogonia of type B.
The spermatogonia (type B) (44 + X+Y) enlarge, to form primary spermatocytes.
The primary spermatocytes (44 + X+Y) now divide so that each of them forms two secondary spermatocytes. This is the first meiotic division: it reduces the number of chromosomes to half.
Each secondary spermatocyte has 22 + X or 22 + Y chromosomes. It divides to form two spermatids. This is the second meiotic division and this time, and there is no reduction in chromosome number(spermatocytogenesis).
Each spermatid (22 + X or 22+Y) gradually changes its shape to become a spermatozoon. This process of transformation of a circular spermatid to a spermatozoon is called spermiogenesis/spermateleosis.
Formation of Sperm
An adult male produces over 1012 to 1013 sperm cells each day. These gradually move into the epididymis and the first portion of the vas deferens, where they undergo further maturation and are stored.
In spermiogenesis first, all nuclei of spermatid shift at one side. Except for chromatin material, all the structures come out from the nucleus. As a result, the nucleus becomes small and light in weight. Then several Golgi vesicles are gathered just above the nucleus.
Some of the Golgi vesicles develop granules called Proacrosomal granules, such granule containing vesicle is called Proacroblast. The rest of the Golgi vesicles are called Golgirest, and the goldcrest dissolves in the cytoplasm. All the proacroblast fuse and form a large vesicle called Acroblast and its granule is called Acrosomal granule.
Acroblast arranged just above the nucleus, then cytoplasm of the spermatid starts moving towards the posterior side. As a result, the plasma membrane shrinks and gets attached to the acroblast and nucleus. Due to this head of sperm is formed.
Now centriole of the spermatid is arranged at a right angle to each other just beneath the nucleus and the distal centriole starts forming axoneme, then all the mitochondria of the spermatid are arranged in spiral order around the axoneme. In this way, a middle piece of sperm is formed. Now axoneme elongates towards the posterior side and forms a tail.
The cytoplasm of the spermatid continuously flows towards the posterior side. At last, cytoplasm constricts from the base and detaches from sperm in the form of large mass and mass of cytoplasm received by Sertoli cells through the method of phagocytosis and provide nutrition to sperm.
Question 3:What is the correct sequence of sperm formation?
Spermatogenesis is the process of formation of haploid spermatozoa (sperms) from diploid spermatogonia inside the testes of the male.
At sexual maturity, the undifferentiated primordial germ cells divide several times by mitosis to produce a large number of spermatogonia or sperm mother cells.
Each spermatogonium actively grows to a larger primary spermatocyte by obtaining nourishment from the nursing cells. The phenomenon of the formation of primary spermatocytes from spermatogonia is called spermatocytogenesis.
Each primary spermatocyte undergoes two successive divisions, called maturation divisions. The first maturation division is reductional or meiotic. Hence, the primary spermatocyte divides into two haploid daughter cells called secondary spermatocytes. Both secondary spermatocytes now undergo second maturation division which is an ordinary mitotic division to form four haploid spermatids, by each primary spermatocyte.
The transformation of spermatids into spermatozoa is called spermiogenesis or spermateleosis or differentiation phase.
Structure of Sperm
The spermatozoon has a head, a middle piece, and a tail. The head is covered by a cap called the Acrosomic cap, Anterior nuclear cap, or Galea Capitis.
The acrosome is a bag-like structure filled with lytic enzymes called Sperm lysins.
In the anterior part of the middle piece, the neck is present. The neck is narrow: It contains a proximal & distal centriole (or Basal body).
An axial filament begins just behind this centriole; it passes through the middle piece and extends into the tail. At the point where the middle piece joins the tail, this axial filament passes through a ring-like structure called the Annulus (or ring centriole or seasons ring).Structure of Sperm
That part of the axial filament which lies in the middle piece is surrounded by a spiral sheath made up of mitochondria (Nebenkern sheath). The nuclear part of the head of spermatozoa consists of chromatin (mostly DNA) that is extremely condensed. It contains a basic nature protein called Protamin.
The axial filament that passes through the middle piece and most of the tail is actually composed of several fibrils arranged. A pair of central fibrils, surrounded by nine pairs (doublets), is arranged in a circle around the central pair (9+2). This arrangement of one central pair of fibrils surrounded by nine doublets is covered by nine solid protein fibres in the middle piece (making the arrangement as 9+9+2).
Only two solid protein fibres cover the proximal tail part (therefore arrangement is 2+9+2), while the end part of the tail has no protein covering (therefore arrangement is 9+2). Immediately outside the fibrils, there is a fibrous sheath.
The middle piece (also called the energy chamber) is surrounded by spirally arranged mitochondria (Nebenkern sheath). Finally, the entire sperm is enclosed in a plasma membrane.
“Oogenesis is the process of formation of female gametes.”
In females, the immature ovum is converted into the mature ovum. This process is called Oogenesis.
In the female ovary, millions of mother cells are formed during fetal development. These mother cells undergo the meiotic cell division and lead to the production of primary Oocytes.
Primary Oocytes are embedded with primary follicles on the outer layer.
Primary follicles are bounded with more granulosa cell layer and from secondary follicles.
Secondary follicles then turn into the tertiary follicle.
At the stage of female puberty, tertiary follicles undergo some structural and functional changes and produce mature Graafian follicle.
Ovum is released from the Graafian follicle during the menstrual cycle. The release of an ovum from the Graafian follicle is called Ovulation.
Ovulation is controlled by the female reproductive hormone, which is stimulated by the pituitary gland.
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